basic type called _norite_, and locally the
ores project beyond the margin into adjacent rocks. Their textures and
their intercrystallization with the primary minerals of the igneous rock
have suggested that they are essentially a part of the norite mass, and
that they crystallized during some segregative processes which were
effective before the magma had solidified. Near the ores there are
likely to be granitic rocks, which, like the ores, seem to be
segregations from the norite magma. Locally both the ores and the
associated granitic rocks replace the main norite body in such a fashion
as to indicate their slightly later crystallization. However, the
intimate association of the ores with the primary minerals in the
magma, together with their absence from higher parts of the norite and
from the extraneous rocks far from the contact, indicate to other
investigators that they were not brought in from outside in vagrant
solutions which followed the intrusion of the main magma, but that they
were segregated within the magma essentially in place. The occurrence of
these heavy ores near the base of the norite naturally suggests that
they were segregated by sinking to the bottom of the molten magma, but
this conclusion implies certain physical conditions of the magma which
have not yet been proved. Again the precise nature of the process and
the part played in it by aqueous and gaseous solutions are subject to
some doubt and controversy. The settlement of this problem awaits the
solution of the more general problem of the origin and crystallization
of magmas.

In this general class of igneous deposits may be mentioned also
diamonds, platinum, chromite, corundum, and other mineral products,
although for the formation of commercial ores of many of these
substances further concentration by weathering and sedimentation has
been required.

Pegmatites are coarsely crystalline acid dike rocks which often
accompany a large igneous intrusion and which have obviously
crystallized somewhat later than the main igneous mass. They may
constitute either sharply delimited dikes or more irregular bodies which
grade into the surrounding igneous mass. They have a composition roughly
similar to the associated igneous rock, but usually a different
proportion of minerals. They are probably the result of the
differentiation of the parent magma. The pegmatites are of especial
interest to the economic geologist because of the frequency with which